THE IMPACT OF INFLUENZA EPIDEMICS ON MORTALITY - INTRODUCING A SEVERITY INDEX

Objectives; The purpose of this study was to assess the impact of rece nt influenza epidemics on mortality in the United States and to develo p an index for comparing the severity of individual epidemics. Methods . A cyclical regression model was applied to weekly national vital sta tistics from 1972 through 1992 to estimate excesses in pneumonia and i nfluenza mortality and all-cause mortality for each influenza season. Each season was categorized on the basis of increments of 2000 pneumon ia and influenza excess deaths, and each of these severity categories was correlated with a range of all-cause excess mortality. Results. Ea ch of the 20 influenza seasons studied was associated with an average of 5600 pneumonia and influenza excess deaths (range, 0-11 800) and 21 300 all-cause excess deaths (range, 0-47 200). Most influenza A(H3N2) seasons fell into severity categories 4 to 6 (23 000-45 000 all-cause excess deaths), whereas most A(H1N1) and B seasons were ranked in cat egories 1 to 3 (0-23 000 such deaths). Conclusions. From 1972 through 1992, influenza epidemics accounted for a total of 426 000 deaths in t he United States, many times more then those associated with recent pa ndemics. The influenza epidemic severity index was useful for categori zing severity and provided improved seasonal estimates of the total nu mber of influenza-related deaths.